1. Field of the Invention
The present invention relates to a service area expansion method for a mobile communication system and a method of processing calls between service areas. More particularly, the present invention relates to a service area expansion method and a method of processing calls between service areas in a mobile communication system adopting a CDMA (code division multiple access) system like a DCS (digital cellular system) or a PCS (personal communication system), in which a reference clock is delayed and an application specific integrated circuit (ASIC) of a plurality of CSMs (cell site modem) in a base station is controlled using such a delayed reference clock so as to expand service area by each CSM ASIC unit, and a handoff area is positioned between expanded service areas so as to perform a normal call processing.
2. Description of the Related Art
In general, as shown in FIG. 1, mobile communication system like DCS or PCS includes a mobile station 1, a base station 2 for transmitting/receiving data to/from the mobile station, a control station 3 for transmitting/receiving data to/from the base station, and an exchange station 4.
A mobile communication system having such a configuration synchronizes timing using a GPS (global position system), and terminals of mobile communications generate signals in accordance with the reference clock.
Thus-generated signals generate a phase delay by a delay in movement distance, and a correct phase tracking is required in demodulating a signal at a receiving terminal.
However, a CSM ASIC which is mounted in a channel card of the base station 2 so as to be used as a channel element has a phase tracking scope of 512PN chip (1PN chip=813 nsec) which can be converted into 125 km, i.e., distance of radio wave movement.
The above-mentioned distance is a round trip distance between the base station 2 and the mobile station 1, and a service area is a radius of 62.5 km from the base station 2 if any hardware or software-related delay does not exist.
The service area in the above-described mobile communication system is restricted within the radius of 60 km due to the limitation of CSM ASIC.
In a conventional art, a pair of CSM ASICs are employed to expand the service area without re-design of CSM ASIC.
That is, the CSM ASIC occupying a normal region within 60 km is capable of performing a phase tracking having a scope of 0 to 512PN chip, and the CSM ASIC occupying an extended region of 60 km to 120 km is provided with a reference clock delayed by 512PN chip and capable of performing a phase tracking having a scope of 512PN to 1,024 PN chip, thus expanding the service area.
As described above, a conventional art expands an overall service area by means of dividing the service area into a normal area and an extended area. However, a method of normally processing calls between service areas is required for cases where a mobile terminal moves from the normal area to the extended area, or from the extended area to the normal area.